1. Field of the Invention
This invention relates to the loading and storage of cryogenic fuel inside a vehicle, and more particularly to the transfer of cryogenic fuel from a main tank to another tank inside of the cargo bay of a vehicle during flight.
2. Description of the Related Art
Current designs for upper stage reusable launch vehicles, satellites and other uses for stored cryogenic fuel call for locating cryogenic tanks inside a closed cargo bay of a launch vehicle. A significant cryogenic liquid leakage during tank chill and fill can result in an unacceptable payload bay overpressurization resulting in a potential loss of the vehicle and the payload. Even a small component leakage in the cargo bay can represent an unacceptable risk of explosion or fire. The risk of leakage is very high during loading operations and during abort landings.
The safety and operations impact of a cryogenic upper stage for the Space Shuttle was extensively evaluated in the Shuttle/Centaur program at NASA. The program was eventually canceled because of "major unresolved safety issues." The safety issues associated with a typical cryogenic upper stage can be grouped into three main areas; loading, mission abort, and post abort landing. The loading issues included; cryogenic component leakage, cargo bay over pressurization, liquid air/nitrogen condensation on the LH.sub.2 tank multi-layered insulation and potential damage to structure, premature separation of the lift-off fill and vent umbilical resulting in the inability to drain and vent the tank, and T zero umbilical/vehicle separation and clearance. The safety issues during mission abort include; the need to rapidly dump both LO.sub.2 and LH.sub.2 in a positive and/or zero g environment, reducing the post dump residuals to a low level prior to 160 k ft altitude, and repressurization of the storage tanks prior to landing. The post landing abort issues include; leakage of hydrogen and oxygen into the closed cargo bay without the presence of a purge gas, venting of hydrogen gas after a landing which can result in unacceptable accumulation of unburned hydrogen outside and inside the vehicle, potential detonation of the unburned hydrogen, unacceptable plume heating of the vehicle vertical tail, and safing the cryo upper stage.
The propellant dump and vacuum inerting issues can be resolved through proper tank and dump system design. However, the loading, and prelaunch issues are more difficult and can only be safely resolved if cryogens are not present. The success of a cryogenic upper stage therefore depends on the ability to safely and reliably load the tanks after lift-off.